Transition-State Structure for the ADP-Ribosylation of Recombinant Giα1Subunits by Pertussis Toxin†
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Pertussis toxin ADP-ribosylates a specific Cys side chain in the alpha-subunit of several G-proteins. Recombinant Gialpha1-subunits were rapidly ADP-ribosylated in the absence of betagamma-subunits, with a Km of 800 microM and a kcat of 40 min-1. Addition of betagamma-subunits decreases Km to 0.3 microM with little change of kcat. Kinetic isotope effects established the transition-state structure for ADP-ribosylation of Gialpha1 subunits. The transition state is dissociative, with a 2.1 A bond to the nicotinamide leaving group and a bond of 2.5 A to the sulfur nucleophile. The nucleophilic participation of Gialpha1 at the transition state is greater than that for water in the hydrolysis of NAD+by pertussis toxin. Crystal structures for Gialpha1 show the Cys nucleophile in a disordered segment or inaccessible for attack on NAD+. Therefore, transition-state formation requires an altered Gialpha1 conformation to expose and ionize Cys. The transition state has been docked into the crystal structure of pertussis toxin in a geometry required for transition state formation.